Typically, an electronic tuner is an apparatus for selecting one signal from among a large number of signals sent from TV stations to receive a desired program.
Referring to FIG. 10, the following will explain the electronic tuner. FIG. 10 is a block diagram showing a typical electronic tuner.
As shown in FIG. 10, receiver signals supplied from an RF-IN terminal of a station-selecting section are first screened to pick out only a desired signal by a band-pass filter 101 (BPF (1)) which operates in accordance with a tuning voltage from a power source 106. Thereafter, the desired signal goes to an AMP 102 to be amplified. Next, the amplified receiver signal is further screened by a band-pass filter 103 (BPF (2)) which operates in accordance with a tuning voltage from the power source 106, and goes to a mixer circuit 104 (MIX).
The mixer circuit 104 is connected to a local oscillator circuit 105 (VCO) which can change an oscillation frequency with a tuning voltage from the power source 106. This local oscillator circuit 105 oscillates a local oscillation signal (LOCAL) in accordance with a tuning voltage and outputs it to the mixer circuit 104.
The mixer circuit 104 performs frequency conversion by mixture of the receiver signal with the local oscillation signal and outputs the frequency-converted receiver signal to an IF-OUT terminal.
This signal outputted to the IF-OUT terminal is referred to as IF signal. The frequency of this IF signal is determined by the difference in frequency between the local oscillation signal and the receiver signal.
Therefore, since the frequency of local oscillation signal is changed in proportion to the desired signal supplied from the RF-IN terminal, the frequency of IF signal is always constant.
Thus, the frequency of the IF signal is always constant, so that it is possible to select only a particular station and receive a program from the selected station.
Incidentally, since VHF used as frequencies for TV stations usually has a wide range of frequencies, an adopted tuner is an electronic tuner of a circuit arranged such that signals are received from the two-divided bands of frequencies. A lower band and upper band of the frequencies are referred to as VHF Low band and VHF High band, respectively.
Therefore, the electronic tuner has control source's lines for the respective bands. This makes possible to receive signals in a broad frequency coverage of VHF in such a manner that in the case where the electronic tuner receives the VHF Low band, a voltage of a VHF Low control source turns High and a circuit of the VHF Low band operates, and in the case where the electronic tuner receives the VHF High band, a voltage of a VHF High control source turns High and a circuit of the VHF High band operates.
FIG. 11 is a block diagram showing a circuit arrangement of a stage before an AMP section shown in FIG. 10.
For example, an electronic tuner shown in FIG. 10 corrects a gain of VHF High band using a circuit (gain correction circuit 107) shown in FIG. 11.
Specifically, in the gain correction circuit 107 shown in FIG. 11, in the case where a voltage of the VHF High control source turns High, a voltage runs through R1 and a voltage appears at a cathode D1, which brings the cathode D1 into conduction. At this time, since the AMP has some input capacity, the circuit in FIG. 11 can be expressed equivalently as a circuit as shown in FIG. 12.
Here, when L1 is set to a proper value, filters occur at L1 and Ci, which can hold up a lower part of the gain. In other words, it is possible to flatly correct frequency characteristic inside a band (VHF High) by holding up a lower part of the gain.
Thus, the electronic tuner includes the foregoing gain correction circuit in some kind of form to improve the frequency characteristic at the VHF High band.
Further, the electronic tuner shown in FIG. 10 is subjected to IF interference described below, and deterioration of frequency characteristic, i.e. deterioration in performance of frequency characteristic occurs even in the VHF Low band.
It is desirable that the IF signal outputted from the IF-OUT terminal of the electronic tuner is a signal from the selected particular station. However, there is the possibility that other interfering signals which come in the electronic tuner due to various kinds of factors might cause deterioration in performance of frequency characteristic.
A typical example of factors of deterioration in performance of frequency characteristic is IF interference. This is an interference that in the case where an interfering signal having the same frequency as the IF signal exists in the RF-IN terminal, the interfering signal directly or indirectly passes through the circuit inside the electronic tuner and is outputted to the IF-OUT terminal, which deteriorates the signal for the selected particular station. This IF interference tends to occur at low frequencies, that is, frequencies close to those of IF signal (VHF Low band).
In order to improve the IF interference, for example, Japanese Utility Model Application Laid-Open Publication No. 105723/1992 (Jitsukaihei 4-105723, published on Sep. 11, 1992) discloses a technique of providing a trap circuit with at least one stage (variable trap circuit) inside the electronic tuner.
However, as described above, since the electronic tuner has been already provided with the gain correction circuit for the improvement of frequency characteristic at the VHF High band, provision of the trap circuit for the improvement of IF interference at the VHF Low band results in a large circuit configuration and price increase of the apparatus. Especially, for the proper improvement of IF interference, it is necessary to increase the number of stages in the trap circuit. Such a case results in a further large circuit configuration and further price increase of the apparatus.